The invention relates to preparing carbonaceous materials suitable for use as electrode compositions.
Lithium-ion cells frequently include a lithium intercalation compound, e.g., LiCoO.sub.2, as the positive electrode and pyrolyzed carbonaceous materials such as carbon or graphite as the negative electrode. Lithium cells, which are used to evaluate properties (e.g., reversible and irreversible capacity) of carbonaceous materials, include lithium metal as the negative electrode and carbonaceous materials as the positive electrode. The pyrolyzed microporous and hard carbonaceous materials that are used in lithium and lithium-ion cells tend to have a high irreversible capacity.
In a lithium-ion cell, where discharge corresponds to removing lithium atoms from the carbonaceous material and charge corresponds to inserting lithium atoms into the carbonaceous material, irreversible capacity is a measure of the amount of lithium that cannot be fully recovered after the first charge (i.e., the amount of lithium that is irreversibly consumed). In a lithium cell, where discharge corresponds to inserting lithium into the carbonaceous material and charge corresponds to removing lithium from the carbonaceous material, irreversible capacity is a measure of the amount of lithium that cannot be fully recovered after the first discharge.
There are a number of mechanisms by which lithium can be consumed. For example, lithium can react with electrolyte at the carbon surface to form a "solid-electrolyte surface." It is theorized, additionally, that pyrolysis produces carbon atoms having sites available for reacting with air to form species, e.g., covalently bonded functional groups, chemisorbed species and physisorbed species.
Numerous efforts have been made to decrease the high irreversible specific capacity of pyrolyzed microporous carbonaceous materials, including conducting the pyrolysis process under inert atmospheric conditions such as, e.g., argon gas, helium gas, nitrogen gas, and under vacuum.